The present invention relates to low-speed, high-torque gerotor motors, and more particularly, to such motors of the type having a separate valve drive shaft driving the valve member.
A typical gerotor motor of the type to which the present invention relates includes a housing defining inlet and outlet ports, and a gerotor gear set. The typical motor further includes valve means to provide fluid communication between the ports and the volume chambers of the gerotor gear set. The invention is especially advantageous when used in a device wherein the gerotor set includes an orbiting and rotating externally-toothed star member, and will be described in connection therewith.
In most gerotor motors, an externally-splined main drive shaft (dogbone) is used to transmit torque from the orbiting and rotating star member to the rotating output shaft. A gerotor motor having a "separate" or "two-piece" valve drive is one in which the valve member is disposed "behind" the gerotor, i.e., at the end of the motor opposite the output shaft, with the output shaft typically being considered the "forward" end of the motor. Conventionally, in such motors, the valve is driven at the speed of rotation of the gerotor star member by means of a valve drive shaft which is in splined engagement with both the valve member and the gerotor star member. See for example, U.S. Pat. No. 4,992,034, assigned to the assignee of the present invention.
In most gerotor motors, the limitation on the torque-transmitting capability of the motor is the strength of the spline connection between the star member and the dogbone. In motors using two-piece valve drives, a portion of the axial length of the splines defined by the gerotor star member is required, merely to drive the valve drive shaft. Driving the valve member, whether it be a disk valve or a spool valve, requires a very small percentage of the total torque output of the motor, but typically, the spline connection between the star member and the orbiting and rotating valve drive shaft has taken up a significant portion of the gerotor star splines. This becomes a more serious problem in the case of relatively small displacement motors, in which the axial length of the gerotor may be on the order of one-half inch.